Filling the System, Bleeding air & Expansion vessel related question

[djahawes]

Hi everyone, wondered if anyone can help.
My brain just recently went into a spaghetti like confusion while I was calmly trying to work this through...

I have recently replaced a faulty expansion vessel and decided at the same time to put antifreeze in for the winter so I'm not at the point of refilling the system.

As I refill the system I obviously need to bleed the air (my bleed valve is at the highest point).

My question is how do I know when:
1. the system is full of water with,
2. no air left in the system AND
3. the Expansion Vessel has not filled with water.

I feel like there is a simple answer and I'm being really stupid but it seems to me that the water I push in to the system to purge further air out will simply fill the Expansion vessel until capacity. The pressure gauge is no help to me because I pressume this will rise to the pressure equal to the pre-charge on the EV and the continue to show this pressure (P*V=C) until the EV is full, then rise to the 3bar limit on the Pressure relief valve.

Do I some how need to purge my air at a system pressure upto but not exceeding the pre-charge on the EV so as not to introduce any water into the EV and once I'm confident there is no more air in the system push the system up just until it equals the pre-charge on the EV (which incidentally is 1.5bar, not that I think this matters for this question).

Any help appreciated.
David

[djahawes]

Ok, just to update and add another question...

I filled the system to 1bar and bled it, topping back up to 1 bar (below the pre-charge of the EV).

I'm assuming I can run it at 1 bar and as the pressure rises at 1.5 bar it will take capacity in the EV.

I think has solved it.


My next question is related to Anti Freeze ratio.
I'm using Alphi 11 at 30% mix and made up enough to fill the system plus some to compensate for amount lost through bleeding.
It seems I underestimated the amount required to bleed out the seemlingly endless air in the system so have had to hook up the mains filler loop to continue the process.
This has in turn reduced my ratio of Antifreeze as I have displaced my AF mix with just water. Rough calculations estimate my 30% ratio is now down to 15%.

Perhaps I should have topped up my hoselock barrel with AF mix instead of using the mains filler loop. Does anyone have any ways they have overcome this problem.
I dont want to run my hoselock dry and end up pumping further air into the system. I was also concerned half way through this process in swapping from hoselock to mains filler knowing there would be air in the pipe.
Ideally I could do with an air purge in the filler tube -Anyone, any ideas?

I think overall of my 3.25Litre system volume I have used approximately 5.75Litres of liquid through filling and bleeding. This seems a lot of volume of essentially wasted fluid for bleeding the air out.

Where am I going wrong please?

Thanks
David

[desperate]

Hallo David, how are you?

I have found by far the most effective way of bleeding air is to first arrange for at least 2 upstands of pipe, or high points where air will tend to collect and then tee in a 15mm gate valve or similar. Then you can rig up a connection to your hoselock pump spray. Fill your sprayer with whatever mix you need, I use 20% with no trouble, pump up your sprayer and open both the gate and sprayvalve until the barrel is nearly empty, as you say do not pump any air in. Refill your barrel and repeat as many times as you need to fill the system. Then with the pump running you can pump fluid into the system and then release the pressure from the sprayer and let your EV pump the fluid and air back into the barrel. You have complete control of the air and fluid with the pump and valves, and  the dip pipe in the barrel naturally separates out the air. Repeat this process a few times at each gate valve...................hey presto no more air. Simply purging through the system whether by the sprayer or filling loop is bound to leave air in various places where the flow will not reach.

It,s a lot easier to do than describe, luckily.

Good luck mate,

Desp

[SimonHobson]

My question is how do I know when:
1. the system is full of water with,
2. no air left in the system AND

No more air comes out of the bleed points, and the pump isn't making gurgling noises are air goes round and round ?

3. the Expansion Vessel has not filled with water.

I feel like there is a simple answer and I'm being really stupid but it seems to me that the water I push in to the system to purge further air out will simply fill the Expansion vessel until capacity. The pressure gauge is no help to me because I pressume this will rise to the pressure equal to the pre-charge on the EV and the continue to show this pressure (P*V=C) until the EV is full, then rise to the 3bar limit on the Pressure relief valve.

Ah, I believe you don't understand how the EV works.

There is a bladder or diaphragm in there to a) separate the air/gas and water, and b) prevent the charge pressure escaping when the system pressure is low (or the EV is not connected to anything).

As you pressurise the system, initially no water will go into the EV - the gas pressure will hold the diaphragm down and prevent water entering. When the water pressure matches the initial charge pressure then that changes. Any further increase in water pressure will cause the gas to be compressed and water will enter the EV - the water will not completely fill the EV.

As you note, P*V = constant. But, you need to work in absolute, not relative pressure. 1 bar on your gauge is actually 2 bar absolute - the 1 bar you are reading, plus the one bar atmospheric pressure. Say your EV was charged to 1 bar, then at the maximum pressure set by the 3 bar relief valve, the volume of the gas will be 2 / 4, or 1/2 of it's original volume - so the EV will be 1/2 full of water.
I think, it's been a long time since science lessons at school

So getting back to your filling. If you know there is still air in the system, you can fill to above normal pressure and put some water into the EV. Then when you open a vent, the EV will push some of that water back into the system. it will help a lot if you can have an assistant to keep topping up the system though.
A tip I heard on another forum (for filling heating systems) was to use a pressure reducing valve (of the same sort as used to lower the pressure on the cold water supply) set to a low value, and fitted with suitable fittings/hose so that it can be used in place of the normal filling loop. You can then just leave the water on while you go round letting air out and the reducing valve will just keep topping the pressure up automatically.